1. Field of the Invention
At least one embodiment of the invention relates to an implantable cardiac therapy device and/or a cardiac monitoring device, such as a cardiac pacemaker or a cardioverter/defibrillator, which is capable of automatically detecting a dislocation of an electrode lead or an electrode pole.
2. Description of the Related Art
Implantable cardiac therapy devices and/or cardiac monitoring devices such as cardiac stimulators in the form of cardiac pacemakers or cardioverters/defibrillators are basically known. Such cardiac stimulators are typically connected to electrode leads that comprise stimulation electrodes and optionally defibrillation electrodes, placed in a ventricle or in the immediate vicinity thereof. Generally, using a stimulation electrode, and more specifically one or more stimulation electrode poles, a cardiac pacemaker may deliver an electrical stimulation pulse to the muscle tissue of a ventricle to thereby induce a stimulated contraction of the ventricle provided the stimulation pulse has sufficient intensity, and the cardiac muscle tissue, the myocardium, is not in a refractory phase during the delivery contraction of a ventricle that was stimulated in this manner is referred to within the scope of this description as a stimulated event. Also, a stimulation pulse that is sufficiently intense to induce a stimulation contraction of a ventricle is commonly referred to as an “above-threshold” stimulation pulse. If a natural contraction of the ventricle occurs, this is commonly referred to as a natural action or a natural or intrinsic event. A contraction of the right atrium of a heart, for instance, is commonly referred to as an atrial event that may be a natural atrial event, for example, or a stimulated atrial event when using an atrial cardiac pacemaker. Generally, a distinction may be made between natural and intrinsic events, and stimulated left ventricular events and stimulated right ventricular events.
Typically, local excitation of the myocardium propagates from the excitation site by conduction in the myocardium, resulting in depolarization of the muscle cells and thus contraction of the myocardium. After a brief period of time the muscle cells are typically repolarized and the myocardium therefore relaxes. During the phase of depolarization, the cardiac muscle cells are insensitive to stimulation, i.e. they are refractory. This period is commonly referred to as the refractory period. The electrical potentials associated with depolarization and repolarization may be sensed, and the variation thereof over time—referred to as an electrocardiogram, which can be evaluated.
Generally, in an electrocardiogram, action potentials that accompany a contraction of the ventricle and reflect depolarization of the cardiac muscle cells, are detected as a Q peak, while the repolarization of the cardiac muscle cells that accompanies the relaxation of the myocardium is reflected in a T wave.
The cardiac rhythm of a healthy individual is typically determined by the sinoatrial node that is controlled by the autonomic nervous system, and that stimulates the right atrium of a human heart and furthermore, via the AV node, the (right) ventricle of the heart. A natural cardiac rhythm originating in the sinoatrial node is commonly referred to as a sinus rhythm, and induces natural contractions of the particular ventricle, which may be detected as natural (intrinsic) events.
The natural (intrinsic) events are most commonly detected by determining the electrical potentials of the myocardium of the particular ventricle using sensing electrodes that are part of a corresponding electrode lead. The sensing electrode poles may also act as stimulation electrode poles, and may be used as a stimulation electrode pole and as a sensing electrode pole. Typically, two adjacent electrode poles may form a pair of sensing electrode poles. For example, a tip electrode and a ring electrode may be provided for sensing, such as sensing of intrinsic events, wherein the tip electrode is also used as the stimulation electrode pole. From the pair of sensing electrodes, a bipolar recording of an intracardial electrocardiogram (IEGM) may be obtained. In that case, sensing and stimulation take place in the ventricle using a ventricular electrode lead, and stimulation and sensing take place in the atrium, such as in the right atrium, using an atrial electrode lead that is separately connected to the cardiac stimulator. In addition, for example, a left ventricular electrode lead may be provided that typically extends via the coronary sinus and a lateral vein branching off of the coronary sinus and into the vicinity of the left ventricle. The left ventricular electrode lead may comprise a stimulation electrode and/or sensing electrode having a small surface area.
With respect to the terms used herein, it is noted that, within the scope of the invention, the terms stimulation electrode or sensing electrode may include a particular electrode pole at an electrode lead, wherein a part of an electrode delivers stimulation pulses and/or receives electrical potentials. It should also be pointed out that it is also common to refer to an electrode lead used for stimulation as a “stimulation electrode”.
During operation of the cardiac stimulator, the sensing electrode poles may be connected to appropriate sensing units, that are designed to evaluate a particular electrocardiogram recorded using a sensing electrode pole, or using a pair of sensing electrode poles, and, in particular, to detect intrinsic atrial or ventricular events such as natural atrial or ventricular contractions. Typically, this takes place, for example, by comparison with a threshold value, wherein an intrinsic event is detected when a particular intracardial electrocardiogram exceeds a suitably specified threshold value.
If an electrode lead, including the sensing electrode poles thereof, should become dislocated, such as “slip” or “move”, generally the amplitude and/or shape of signals obtained using the one or more sensing electrode poles may change, albeit not for physiological reasons. This poses a problem with respect to the reliable evaluation of signals that are recorded, and it is therefore desirable to detect a dislocation of an electrode lead and/or the electrode poles thereof, especially since a number of relevant therapy parameters may be derived from recorded signals.
On the basis of the frequency at which the atrial and ventricular events follow one another, the particular intrinsic atrial heart rate, such as an atrial frequency, or ventricular heart rate, such as a ventricular frequency, may be derived, thus enabling detection of tachycardias.
In the case of known demand pacemakers, the detection of natural events is also used to suppress, or inhibit, the delivery of stimulation pulses to a particular ventricle if the natural event is detected within a time window before the planned delivery of a stimulation pulse to the ventricle occurs. As common in the art, using rate-adaptive cardiac pacemakers, the point in time for delivery of a particular stimulation pulse is planned depending on a particular stimulation rate which should correspond to a patient's physiological demand, that is typically higher when exertion is greater, for instance. For this purpose, a cardiac stimulator may be equipped with one or more activity sensors, such as a Closed Loop Stimulation (CLS) sensor, which is described in greater detail below.
With respect to a dislocation of sensing electrode poles that may influence the sensing of events, approaches for detecting such a dislocation are known. Known approaches for detecting a dislocation of a stimulation electrode are based on the evaluation of the sensing amplitude, the electrode impedances and the pacing thresholds.
For example, U.S. Pat. No. 7,664,550 to Eick et al., entitled “Method and Apparatus for Detecting Left Ventricular Lead Displacement Based Upon EGM Change”, appears to disclose a method for detecting the displacement of a left-ventricular electrode lead and the electrode poles thereof, by way of a changed signal amplitude, morphology or a changed time interval from an atrial signal.
Typically, the methods for detecting an electrode dislocation used with a left-ventricular electrode lead, such as a coronary sinus electrode lead, are limited since the electrode lead is implanted in a vein. As such, if dislocation occurs, it is often displaced within the vein by “only” a few millimeters to a few centimeters. In one or more embodiments, the parameters “signal amplitude”, “impedance” and “stimulation threshold” may be influenced only slightly, thereby making it impossible to reliably detect such a dislocation.
The stated disadvantages of the methods that have been commonplace so far are basically solved by the methods of the IEGM signal comparison proposed in Eick '550. However, the methods of signal analysis presented in Eick '550 are not sensitive enough to reliably distinguish normal IEGM excursions, such as different heart rates and extra systoles, from an actual electrode dislocation. In addition, it appears as though the system of Eick et al. does not make it possible to determine the extent of the dislocation and to adapt the CRT stimulation accordingly.